The present invention concerns a pressurized-water reactor whose primary cooling system gives off its heat within a pressure vessel to a steam generator within the vessel and which is inserted between the reactor core and the intake space of the circulating pumps, i.e., in the cold leg of the main coolant loop. This pressurized-water reactor, whose primary cooling system is normally operated with forced circulation, is particularly well suited for ship propulsion and stationary installations of smaller power rating. In order to avoid damage to the circulating pumps of the primary cooling system due to cavitation, particularly in the case of rapid load changes, these pumps should operate in a zone as cool as possible, of the primary cooling system. If these pumps operate at close to the boiling temperature of the pressurized-water coolant, a small brief lowering of the normal pressure is sufficient to cause the water in the pump housings to boil. The steam bubbles produced in this manner collapse during any subsequent pressure increase and cause destruction at the pump internals in a locally closely confined area. Particularly exposed in this respect are circulating pumps which, for reasons of better maintenance and inspection, are preferably located above the core and steam generator, because as compared to pumps located underneath, such pumps receive a considerably smaller inflow head or hydrostatic pressure and therefore have a greater tendency toward cavitation. These problems are often avoided by means of an external pressurizer for the coolant which, however, requires much space and is expensive.
A typical example for a shipboard reactor proven in practice is represented by the propulsion plant of the nuclear vessel "OTTO HAHN". In this arrangement, with a pump located at the bottom of the pressure vessel, the desired flow can be realized with water as cold as possible ahead of the pump and with a considerable inflow head to the pump. If, with the design the same otherwise, the circulating pump or pumps are arranged at the upper end of the pressure vessel, e.g., in its closure head end, the pumps not only are approached by the hotter primary coolant, but the inflow head is at the same time also reduced considerably.
In the disclosures of the German Offenlegungsschrift No. 22 27 895 (U.S. Ser. No. 153,304, filed June 15, 1971 now U.S. Pat. 3,888,734) the described problems with the circulating pumps are avoided by using a very large and, therefore, also expensive external pressurizer. In this external pressurizer, the pressure in the pressure vessel is adjusted to the desired level by controlled heating of the water in the pressurizer or injection of cold water. In the same disclosures, it is proposed to support all the internals in the pressure vessel at the vessel's upper closure head, so that for every refueling and every inspection or maintenance action on internals in the pressure vessel, practically all the internals must be removed together from the pressure vessel, with their considerable weight. Such refueling is necessary about every one to two years, while a routine inspection is necessary only every 8 years. It is therefore desirable that the fuel assemblies can be exchanged without the necessity to remove the steam generator.
In order to obtain the desired flow path from the reactor through the heat exchanger to the circulating pump, it is proposed in the same disclosures to have the flow through the reactor core go in normal operation from top to bottom. This flow path results, in the case of load changes or at low load as well as in the event of a failure of the circulating pumps, in very confusing flow conditions in the reactor core which should be avoided.
An object of the present invention is to provide a pressurized-water reactor whose primary cooling system gives off its heat within a pressure vessel, to a steam generator which within the pressure vessel is inserted between the reactor core and the suction space of the circulating pumps, and which can be disassembled without destruction for inspecting the pressure vessel.
A specific object of the present invention is to provide an arrangement of a steam generator which can take up the decay heat of the reactor core through natural circulation in the event of a failure of the circulating pumps and lowering of the water level.
A further object of the present invention is to provide a pressurized-water reactor whose internals in the pressure vessel, fabricated from austenitic metal, can expand freely relative to the pressure vessel, which is made of ferritic metal.